Method of and apparatus for generating aerosols by electric arc

ABSTRACT

An electric arc struck between a counterelectrode connected to the anode of a source of current, and a material to be sampled connected to the cathode, causes the ejection of very small droplets of the material. The droplets solidify and are carried away as an aerosol by the gas used to sustain the arc. The droplets are representative in their composition of the entire region of the material struck by the arc.

United States Patent [72] Inventors Ralph Leon Dahlqulst 50 Field 6:Search 356/85, 87, Sam Barbara; 74, 36; 313/231;219/121, 271, 192;204/249 James Latimer Jones, Santa Barbara; Kenneth William Paschen,061m, all of, 1 References CI'ed m, UNITED STATES PATENTS Appl. No.737,252 3,188,180 6/1965 Holler 356/86 Filed May 0,1968 3,217,16211/1965 Wehner... 356/85 at s- 3 1971 3,304,402 2/1967 Thorpe... 313/231x Assignee pp Research Laboratories, 3,325,976 6/1967 West 356/85 Sunand, C 3,402,31 1 9 1968 Fitzgerald 356/86 C n -p of avplimfion3,467,471 9/1969 Greenfield et al. 356/85 644,987, June 9, 1967, nowabandoned.

PrimaryExaminer-Ronald L. Wibert Assistant ExaminerV. P. McGraw Att0rneyHoffman Stone METHOD OF AND APPARATUS FOR AB T C l t t GENE A I AEROSOLSBY ELECTRIC ARC S RA T An e ec r1c arc struck between a coun erelectrode connected to the anode of a source of' current, and a 16 Claims 8Drawing material to be sampled connected to the cathode, causes the US.Cl 356/36, ejection of very small droplets of the material. The droplets356/74, 356/86, 313/231 solidify and are carried away as an aerosol bythe gas used to Int. Cl G0ln 1/00, sustain the arc. The droplets arerepresentative in their composition of the entire region of the materialstruck by the arc.

PATENTED M1831 I97l SHEET 1 UF 2 ow /Ayn" 4 INVENTORS JAMES L. JONESKENNETH W. PASCHEN RALPH L. DAHLQUIST ATTORNEY PATENTED AUG31 I97!3302595 SHEET 2 OF 2 SOURCE SOURCE I MAT MAT'L. POTENTlAL POTENTIAL--4oo,usEc.

300,44 SEC. H6 7 FIG. 4

' -40AMPERES L-30-4O M SEC.

F IO M SEC.

FIG. 5

SOURCE MAT'L. POTENTIAL U CE [1 K- t A A A A A -soo,q SEC. POTENTIAL VUV HG 8 INVENTORS 21k; mi mm K N woo/a SEC. RALPH L. HLQUIST FIG. 6 BYATTORNEY METHOD OF AND APPARATUS FOR GENERATING AEROSOLS BY ELECTRIC ARCBRIEF SUMMARY This application is a continuation-in-part of the pendingapplication of the same inventors, Ser. No. 644,987, filed June 9, 1967,entitled Method of and Apparatus for Generating Aerosols by Electric Arcto Obtain Samples for Chemical Analysis, and assigned to the presentassignee now abandoned.

This invention relates to a novel method of and apparatus for nebulizinga material to obtain a sample for chemical analysis or the like, or forany other purpose when it is desired that the composition of thenebulized material be closely representative of the average compositionof a reasonably large region of the body from which it is taken. Y

The invention arose in connection with efforts to improvespectrochemical analytical methods, and its background and advantageswill be described herein primarily with respect to spectrochemical work.It is expected, however, that the invention will also be of significantvalue for other purposes such as for use in preparing samples for othermethods of chemical analysis including wet processes, and for obtainingfine powders for any purpose where it is desired that the powders becomposed of extremely small particles, or that the composition of thepowders be closely representative of the composition of a fairly largeregion of the material from which they are taken.

spectrochemical methods of analysis are widely used and have been foundto be especially advantageous for process control, largely because ofthe high speed with which analyses can be made by these methods. Forexample, the manufacture of steel can be more closely controlled if thecomposition of the heat is known at the time it is poured. Previously tothe adoption of spectrochemical methods, it was regarded asimpracticable to hold a heat pending completion by wet chemical methodsof an analysis of a sample taken from it. Not only were the fuel,equipment and labor utilization costs regarded as intolerable, but alsothe composition was apt to change significantly during the timerequired. With spectrochemical methods as heretofore practiced, it hasbeen possible in most cases to obtain fully adequate analyses of samplesin less than ten minutes, thus making highly accurate compositioncontrol fully feasible.

The present invention arose out of efforts both to, reduce the timeneeded for analysis still further and to improve the quality of thesamples so that they would be more truly representative of the actualaveragecomposition of the heat than samples heretofore obtainable. Theadvantages of increased speed are obvious and need not be discussedherein. The problem of compositional representativeness, however, ismore subtle.

It is generally recognized that for samples prepared by methods that donot include the step of preparing a liquid solution, the accuracy ofspectrochemical analysis is limited by the homogeneity andrepresentativeness of the portion of the sample that is actually excitedto produce radiation during the analysis. In optical emission analysis,for example, a spark discharge to the solid metal sample often vaporizesless than a milligram of metal, and even though the composition of thesample taken as a whole may be accurately representative of thecomposition of the entire heat, segregation of constituents duringfreezing creates substantial variations in the compositions of evenfairly closely spaced regions of the sample. By adequate stirring andmixing in the melt, it-is easily possible to achieve a sample body,which, on the whole, is representative in composition of the entiremelt. Homogeneity of the sample, however, is a much more difficultcondition to achieve. Various different constituents of the sample tendto segregate as the sample freezes, so that casting procedures, forexample, cannot be expected to provide homogeneity down to themicroscopic scale desirable for optical emission analysis.

in X-ray fluorescence analysis, much larger areas of the surface of asolid sample may be excited than are utilized in optical emission.Typically, 4 or 5 square centimeters may be irradiated, but thefluorescent X-rays are produced from surface layers of only one to a fewhundred microns in thickness depending upon the particular analytesselected and the matrix in which they are held. Again, the analysis isbased on amounts of materials of but 1 to milligrams at the most, eachelement is determined from a layer of different thickness, and theaccuracy of the analysis depends upon the accuracy with which thecomposition of a relatively thin surface layer of the sample correspondsto the average composition of the entire melt.

In analysis by electron microprobe, an electron beam, usually less than1 micron in diameter, is directed upon the sample to generate X-rays,which are then spectrometrically analyzed. The volume of the materialinvolved in analysis of this type is limited to the volume required tostop the electrons of the beam. This is typically a few cubic microns.With the electron microprobe it has been shown that in the solid samplesnormally used for analysis by optical emission or by X-ray fluorescence,regions spaced only a few microns from each other are of significantlydifferent compositions.

The practice of the invention not onlyenables a reduction in the timeneeded for preparing samples and presenting them to the spectrometricapparatus, but also overcomes difficulties of obtaining samples thatare, homogeneous throughout on a microscopic scale. Moreover, due to theway in which the samples are formed, they are more readily dissolvedthan samples made by casting, and thus enable a reduction in timerequired for wet chemical analysis.

Briefly, the practice of the invention contemplates the use of anelectric arc together with a stream of gas to produce. an aerosol from amaterial to be sampled and to carry the aerosol away from the surfacewhere it is produced. The are is struck directly to the, surface of thematerial, and causes the ejection of very fine particles. The current ofthe arc may be controllably varied, and the nature of the gas selectedto achieve optimum particle emission from the material being sampled,both as to quantity and as to the sizes of the individual particles. Inaddition, so long as the material is electrically conductive, there isno effective limitation as to its temperature, and samples may readilybe obtained from molten materials.

The aerosol particles produced in accordance with the present inventionmay readily be made, predominantly 1 micron and smaller in diameter.They may be conducted directly to a plasma flame chamber for immediateanalysis by optical emission, or otherwise analyzed as desired. Theaerosol may be passed through a filter to collect its solid particles.Which may then be analyzed by X-ray spectrometric techniques orotherwise as desired. The collected solid particles, being very finelydivided may also be very quickly the solved for analysis by wetprocesses, so that even for wet analyses, the practice of the inventionenables a significant improvement in speed as well as in homogeneity andrepresentativeness.

The practice of the invention enables the production of a fine metalpowder of a very high degree of homogeneity, which is trulyrepresentative composition-wise of the material from which the aerosoldroplets are ejected.

The material to be nebulized is made the cathode for a DC arc, whichappears to produce cavitation, or some generally similar effect on thesurface of molten materials accompanied by the ejection of fineparticles. Similar action is obtained when the arc is applied to solidsurfaces, apparently accompanied by highly localized melting of thematerial on the surface. The action is presently thought to be caused bya very steep potential gradient immediately adjacent to the cathode.Even when the arc is of low voltage, the potential gradient appears tobe very high at the cathode.

The quantity of particles ejected from the surface has been found todepend upon the selection of the gas used to sustain the arc, which, ifa large flow of aerosol is desired, should be one capable of producing alarge number of positive ions in the arc. The quantity of aerosolproduced also depends upon the flow of gas, which determines the rate atwhich the particles are removed from the region adjacent to the surfaceof the material.

When small bodies of a molten material are to be analyzed of the kindwhere constituents of the material tend to separate in the melt, it isdesirable to utilize supplemental heating of the type that causesadequate stirring, such as, for example, induction heating, or toprovide for stirring in some other way before starting to draw theaerosol from the material.

DETAILED DESCRIPTION Representative embodiments of the invention willnow be described in connection with the accompanying drawing, wherein;

FIG. I is a schematic, cross-sectional view of an aerosol generator inaccordance with the invention arranged for nebulizing a solid material;

FIG. 2 is a schematic, cross-sectional view showing apparatus accordingto a modified form of the invention, as arranged for producing anaerosol from a small body of molten material or from a flowing stream ofmolten material;

FIG. 3 is'a schematic, cross-sectional view of a lance in accordancewith the invention for obtaining an aerosol from a large body of moltenmaterial, such as, for example, a heat of steel in an open hearthfurnace;

FIG. 4 is a chart showingthe arc current produced by a periodic highvoltage discharge;

FIG. 5 is a chart showing the arc current produced by application of aconstant direct current source of fairly low internal impedance;

FIG. 6 is a chart showing the arc current produced by a periodic lowvoltage discharge, with certain reactors in series between the arc andthe discharge source to damp oscillations to a small extent;

FIG. 7 is a chart showing the arc current produced as in the case ofFIG. 6, but with the reactors selected to achieve critical damping; and

FIG. 8 is a chart showing the arc current produced as in the cases ofFIGS. 6 and 7, but with the reactors chosen to produce greater thancritical damping. According to a first'illustrative embodiment of theinvention as shown in FIG. 1, an aerosol 10 is produced from a solid,electrically conductive body 12, and withdrawn through an exhaust tube14 for any desired use. The open tip 16 of the exhaust tube is placedclosely adjacent to the surface of the body 12 to be analyzed, andserves as an anode for striking an arc between the tube 14 and the body12. The tube 14 is preferably of copper, and water cooled, as shown, sothat it does not become heated by the are sufficiently to eject its ownconstituent materials. Its open end 16 is preferably additionallyprotected, as shown, by a centrally apertured cap 17 of highlyrefractory and corrosion resistant, insulating material, which operatesto restrict the arc to the inner wall surface of the tube 14, tostabilize the arc, to distribute its upper end around the innercircumference of the tube 14, and to concentrate its lower end toward aregion on the surface of the body 12 near the central axis of the tube14.

In operation, the cap 17 also produces a jetlike, restrictive effectupon the arc, confining it to a fairly small region on the surface ofthe material being sampled directly opposite the central aperture 19.The effect is thought to be due, at least in part, to the reduction ingas pressure caused by the flow of gas through the aperture 19, which isaccelerated by heating of the gas by the arc itself. The effect may beenhanced by imparting a tangential motion to the gas to create aswirling effeet as it enters the aperture 19, further to reduce thepressure along the central axis of the aperture.

An enclosure 18, which may be of insulating material, as shown, isfitted around the lower end of the exhaust tube 14 to confine theworking gas and prevent its escape except through the exhaust tube 14.Altemately, if desired, the enclosure 18 may be of a conductivematerial, in which case it should be insulated from the exhaust tube 14and of adequate internal diameter to insure against striking of an arcbetween it and the exhaust tube l4.'The working gas, which may,typically, be helium, argon, or nitrogen, is introduced through an inlet20 in the enclosure 18.

In operation, the enclosure 18 and the exhaust tube 14 are first flushedby flowing the working gas through them to remove air and to provide thedesired working atmosphere. The are is then struck by passing amomentary high voltage discharge between the anode l4 and the body 12,and may thereafter be maintained at a low voltage sufficient to maintainan average current of at least about -3 amperes, and preferably lessthan about 50 amperes. The are strikes the surface of the body 12 at avery small point and tends to move rapidly over the surface in what maybe called a random scanning pattern. After a few minutes, the wholesurface of the body 12 beneath the open end of the exhaust tube 14presents an etched appearance. Local melting and sputtering occurwherever the arc strikes the body 12. The arc has a natural tendency toavoid molten portions of the body and to anchor itself to a solidsurface. It is seen to be constantly moving over the surface, therebyproviding successive very small samples from successive differentportions of the body 12, thus insuring that the aerosol is highlyrepresentative in composition of a fairly large region of the body 12.

The working gas continues to flow through the enclosure picking updroplets of the material that are ejected from the surface of the body12, and carrying the droplets through the exhaust tube in the form of anaerosol 10. The droplets freeze rapidly, without coalescing, to form anaerosol of minute solid particles, typically smaller than 1 micron indiameter. The flow of the working gas tends to concentrate the arc andto stabilize it, depending upon the nature of the gas and its flow ratein relation to the physical dimensions of the exhaust tube 14 and itsspacing from the surface of the body 12 under analysis. Maximumconcentration of the arc and the most satisfactory results have beenachieved in the work done thus far by the use of helium, which has beenfound to be effective at much lower rates than, for example, argon.

The embodiment of the invention illustrated in FIG. 2 is intendedprimarily for obtaining aerosols from molten materials. It includes acover 24 of an insulating material enclosing a crucible 26, which hasinsulating sidewalls 27, and a conductive bottom wall 30 to provideelectrical contact with the material 28 in the crucible. The bottom wall30 is preferably water cooled, as shown, for use with materials thatmelt at high temperatures. The crucible 26 may be of any desiredconfiguration. It may, for example, be in the form of an elongatedtrough for conducting a continuous stream of molten mate 2'? ill throughthe sampling zone. As shown, an induction coil 32 is mounted around thecrucible 26 for electromagnetically heating and stirring the moltenmaterial 28. The combination exhaust tube and arcing anode 14 extendsthrough one wall of the cover 24 and terminates adjacent to the uppersurface of the molten specimen material 28.

Operation of this embodiment of the invention is identical in principleto the operation of the first described embodiment herein. There is lessrapid motion of the are over the surface of the material 28, butcompositional representativeness is assured by reason of convectioncurrents and agitation in the melt. Droplets of the material 28 ofmicroscopic size are ejected by the are from the surface of the material28 and are swept into the exhaust tube 14 by the flow of working gas,which enters through an inlet 20in the cover. In this case also, thedroplets freeze rapidly to form an aerosol of solids.

Some of the droplets ejected from the surface of the material 28 maybecome completely vaporized as they pass through the arc, but theresulting vapors recondense very rapidly as they enter the exhaust tube14 because of the cooling effect of the working gas. Insofar as ispresently known, vaporization is not significant in the practice of theinvention, and its occurrence or absence may be ignored.

Generally similar, but usually less satisfactory results may be achievedin this embodiment of the invention without using the induction coil 32to heat the material 28. In cases where it is desired to melt aninitially solid material in the crucible 26, the arc itself may providesufficient heat to melt the entire body, or a portion of it to form apuddle on its surface. In cases where the material 28 is already moltenwhen it is fed into the crucible 26, additional heating may not beneeded. The use of induction heating, however, is preferred, because itpermits better control, and especially because of its stirring effect,which enhances the representativeness of the composition of the aerosolproduced.

The lance shown in FIG. 3 is proposed for use in monitoring on acontinuous or intermittent basis, as desired, the composition of a largemass of molten material such as, for example, a heat in an open hearthfurnace. The lance includes an exhaust tube 40 generally similar to theexhaust tubes 14 shown in the embodiments of FIGS. 1 and 2, butpreferably of more rigid and rugged construction to enable it better towithstand the buffeting it may be subjected to in this type ofenvironment. A lower end portion of the tube 40 of any desired length issurrounded by an enclosure arrangement, which as shown is constituted bya water cooled, cylindrical contact electrode 41. The contact electrode41 extends beyond the open end of the tube 40 and is insulated from thetube 40 by any desired means such as the cup-shaped mounting element 36shown.

Several alternative arrangements are contemplated. For example, thecontact electrode 41 may be in the form of a rod, in which case theinsulating mounting element would constitute the enclosure arrangementand would extend beyond the end of the tube 40, In another construction,the mounting element 36 may be a simple annulus, in which caseinsulation in the form either of a coating or of additional spacingwould be provided between the exhaust tube 40 and the contact electrode41.

The contact electrode 41 extends beyond the lower end of the exhausttube 40 sufficiently far so that when the lance is lowered into themolten bath, only moderate pressure of the working gas will be requiredto keep the surface of the melt 44 spaced away from the open end of theexhaust tube 40. The contact electrode 41 makes a relatively large areaelectrical contact with the melt 44 in a region reasonably close to theexhaust tube 40, thereby minimizing the loss of energy by jouleanheating of the melt.

In operation, the lance is simply lowered into the melt 44 to anydesired depth short of immersing the entire length of the contactelectrode 41, and the working gas is introduced into the annular space42 within the electrode 41. If the surface of the melt 44 iscontaminated as, for example, by slag or an oxide coating, thecontaminants may be swept away and a clean surface provided byincreasing the pressure of the working gas to cause it to escaperadially outwardly from the lower end of the electrode 41.

The pressure is reduced before starting the arc to a value that holdsthe melt below the end of the exhaust tube 40 but does not cause the gasto escape outwardly from the contact electrode 41.

In actual operation with devices of this type, using arc currents offrom 5 to 50 amperes at relatively low voltages, it has been found thataerosols may readily be generated containing at least about 100milligrams of solids per minute. When collected in the form of a filmsuch as, for example, by passing the aerosol through a filter membrane,the solid particles make an excellent sample for X-ray fluorescenceanalysis, and also may be rapidly dissolved for use in wet chemicalprocesses.

It is expected that the process will be found advantageous also for theproduction of powders such as powdered iron in cases where it is desiredthat the particles of the powders be of very small size or of veryuniform composition, or both. The particles produced in the practice ofthe invention are highly uniform in composition, and may easily be madesmaller than 1 micron in diameter, on the average, by suitable controlof arcing current and gas flow rate.

In the practice of the invention, the material from which the aerosol isto be generated is always connected to the negative terminal of thesource of electricity used to sustain the are. In this sense, the termcathodic DC arcing may be used to characterize the invention.

The currents in the arcs, however, have been found to includesubstantial alternating components under all conditions so farinvestigated, and actual current reversals occur, so that with referenceto the currents in the arcs, the terms DC and unidirectional may be verymisleading.

The chart of FIG. 4, for example, shows the damped current oscillationsthat occur in an arc in the practice of the invention during onedischarge of a high voltage spark generator of a conventional type. Thegenerator was set for a repetition rate of 240 sparks per second at18,000 volts. As may be seen, the

arc current rapidly builds up at the beginning of the discharge to avery high value in the direction of the initially applied voltage. Itthereafter rings for the balance of about 300 microseconds at a rate ofabout 1 megahertz.

The chart of FIG. 5 shows schematically the current an are produced inthe practice of the invention in a case wherein the arc was energized bya DC power supply of fairly low internal impedance, set to indicate anominal average output of about 3 amperes. The are current fluctuatedwidely, and included substantial AC components at various frequencies upto at least about 1 megahertz. Accurate measurements of frequency weredifficult to make because of the apparently random variations observed,but from observations of the oscilloscope, it appeared that the highfrequency oscillatory currents, or hash 50, were interrupted from timeto time by trains of unidirectional pulses 52 of current of about 40amperes, each pulse persisting for from 10 to about 40 milliseconds.

The charts of FIGS. 6, 7, and 8 show the arc currents produced by thedischarge of a capacitor through the arc in series with selectedreactors. In each case, the arc was initiated by a very brief highvoltage pulse, and then sustained by current from the capacitor, whichwas charged to 1000 volts at the beginning of each discharge.- Therepetition rate was 60 per second.

In the first case, FIG. 6, the capacitor was 5 microfarads in value, anda 360 microhenry inductor was connected in series with it. The dischargewas underdamped, and the current in the arc oscillated, as shown, atabout 3500 hertz for about 1500 microseconds following the initiation ofthe arc.

In the second case, FIG. 7, the circuit was arranged to produce criticaldamping. The capacitor was 10 microfarads in value. A resistor of 5ohms, and an inductor of 50 microhenries were connected in seriesbetween the capacitor and the arc electrodes. After arc initiation bythe high voltage pulse, the current in the arc responded quickly to thevoltage impressed by the capacitor, and decayed within about 400microseconds without oscillation.

In the third case, FIG. 8, overdamping was provided. The capacitor wasof 30 microfarads, and connected to the arc electrodes through a 3 ohmresistor and a 360 microhenry inductor. In this case also, there was nodetectable reversal of current once the capacitive discharge assumedcontrol after arc initiation.

It is not clearly understood why the cathodic arc produces the improvedresults that have been noted. The currents in the arc seem to be seldompurely unidirectional. It appears that at the cathode, the arc is muchless stable in position than at the anode. It moves across a fairlylarge surface area of the cathode, sputtering material from differentsuccessive incremental areas of it, and producing less intense localizedheating. Both of these effects are believed to contribute to thecompositional representativeness of the aerosols produced.

Movement of the arc insures sampling over a macroscopic portion of thecathode. Lack of intense local heating tends to avoid excessivevolatization and the effects of preferential volatization of the variousdifferent components of the cathode.

The term unipotential source seems to be the most apt one to describethe principal limiting feature of the invention. As used herein it isintended to include not only conventional batteries and direct currentpower supplies, but also high voltage spark generators in which theoutput voltage at the time of spark initiation is always of apredetermined polarity, and repetitive capacitive discharge sources inwhich the output capacitor is always charged in a predetermined polarityat the start of each discharge. in the practice of the invention, thematerial to be nebulized is connected to the nominally negative terminalof the source, and serves as the cathode for the arc current on a timeaverage basis. Although the arc current may reverse momentarily, the netcurrent taken over the arcing period flows from the counter electrode tothe material to be nebulized.

WHAT IS CLAIMED 1S:

1. Method of producing an aerosol comprising passing an electric arebetween a counter electrode and a source material with the net currentflow being in the direction from the counter electrode to the sourcematerial, providing enough energy in the arc to cause droplets of thesource material to be ejected from it to form an aerosol, simultaneouslyflowing a selected gas through the region of the arc to carry theaerosol composed of the gas and droplets ejected from the material awayfrom the source material, the counter electrode being arranged to avoidsputtering from it.

2. Method according to claim 1 wherein the selected gas is selected fromthe group consisting of argon, helium, and nitrogen.

3. Method of producing an aerosol comprising passing an electric arebetween a counter electrode and a source material with the net currentflow being in the direction from the counter electrode to the sourcematerial, providing enough energy in the arc to cause droplets of thesource material to be ejected from it to form an aerosol, simultaneouslyflowing a selected gas through the region of the arc to carry theaerosol so formed away from the material, and to solidify the droplets,the counter electrode being arranged to avoid sputtering from it.

4. Method of producing an aerosol from a molten source materialcomprising passing a direct current electric are between a counterelectrode an an anode and the source material as a cathode withsufficient energy to cause droplets of the source material to be ejectedfrom its surface to form an aerosol, simultaneously flowing a selectedgas through the region of the arc to carry the aerosol so formed awayfrom the material, the counter electrode being arranged to avoidsputtering from it.

5. Method of producing a specimen the composition of which isrepresentative on a microscopic scale of the composition of a fairlylarge region of a solid body comprising passing an electric are betweena counter electrode an an anode and the solid body as a cathode withsufficient energy to cause droplets of the material of the body to beejected from its surface to form an aerosol, simultaneously flowing aselected gas through the region of the arc to carry the aerosol soformed away from the surface of the body, the counter electrode beingarranged to avoid sputtering from it.

6. Method according to claim 5 including also the step of maintaining asufficient current in the arc to melt at least a macroscopic portion ofthe body.

7. Method according to claim 6 including the step of heating the body byelectromagnetic induction and thereby stirring the molten portionthereof.

8. Method of chemical analysis comprising the steps of producing anaerosol in accordance with the method of claim 1, and analyzing thesolids portion of the aerosol so produced.

9. Method of monitoring the composition of a bath of a molten,electrically conductive material comprising producing an aerosol inaccordance with the method of claim 1, conducting it to a spectrometricanalytical device, and analyzing the solid particles of the aerosolspectrometrically.

10. Apparatus for producing an aerosol from a source materi ls t pri i ea. an electrode,

b. enclosure means for enclosing a region adjacent to a selected surfaceof the source material, said enclosure means enabling said electrode tobe positioned within said region and adjacent to the surface of thesource material, said enclosure means also including inlet means andoutlet means,

d. a unipotential source of electric current, e. means for connectingthe source material to the cathode of said current source and saidelectrode to the anode of said current source, and

f. means for flowing a selected gas through said enclosure means viasaid inlet and outlet means while said connecting means is operative, tofacilitate the striking and maintenance of an are between said electrodeand the source material and to carry particles of the source materialdislodged by the are out of said region through said outlet means.

11. A lance for producing an aerosol from a bath of a molten materialcomprising:

a. a tube having an open end for withdrawing an aerosol from a regionadjacent to the surface of the molten material, said tube beingelectrically conductive,

b. means defining an open-ended enclosure around a terminal portion ofsaid tube and extending beyond the end of said tube,

c. means for flowing a gas into the enclosure defined by said enclosuremeans from a point spaced from the open end thereof and outside of saidtube, and thence out of the enclosure through said tube;

d. means for making electrical contact with a molten material at a pointspaced from said tube when said tube with said enclosure means is placedopen and first into the molten material, whereby a source of electriccurrent may be connected between said tube and the source material toproduce an electric arc between them strong enough to dislodge smallparticles of the source material from its surface, and

e. said gas means being operative to carry particles dislodged from thesource material by an are out of the enclosure through said tube.

12. Apparatus in accordance with claim 11 including dynamic coolingmeans to enable the lance to Withstand continued exposure to elevatedtemperatures.

13. Apparatus in accordance with claim 11 including a cap of aninsulating, refractory material covering the open end of said tube forlimiting an arc struck therefrom to the inner wall surface thereof, saidcap having an aperture coaxially aligned with said tube to permitstriking the arc and passage of aerosols into said tube.

14. Apparatus in accordance with claim 11, wherein said electricalcontact means is a cylindrical conductive member and constitutes theouter wall portion of said enclosure means.

15. Apparatus for producing an aerosol from a source materialcomprising:

a. a tubular electrode open at one end, and having an exhaust openingspaced from said one end,

b. means for supporting said electrode with said open end adjacent toand spaced from a source material,

c. enclosure means for enclosing a region adjacent to the sourcematerial and including said open end of said electrode,

d. gas flow means for introducing a carrier gas into the region enclosedby said enclosure means and withdrawing it from the region through theopen end of said electrode,

e. an unipotential source of electric current,

f. means for connecting the source material to the cathode of saidcurrent source and said electrode to the anode thereof, and

g. said gas flow means being effective to sweep small parti cles such asmay be produced by an arc energized by said current source out of saidenclosure means through said electrode.

16. Apparatus according to claim 15 including an annular insulatingshield fixed to the open end of said electrode for confining an arcstruck from said electrode to the internal surface thereof.

1. Method of producing an aerosol comprising passing an electric arcbetween a counter electrode and a source material with the net currentflow being in the direction from the counter electrode to the sourcematerial, providing enough energy in the arc to cause droplets of thesource material to be ejected from it to form an aerosol, simultaneouslyflowing a selected gas through the region of the arc to carry theaerosol composed of the gas and droplets ejected from the material awayfrom the source material, the counter electrode being arranged to avoidsputtering from it.
 2. Method according to claim 1 wherein the selectedgas is selecTed from the group consisting of argon, helium, andnitrogen.
 3. Method of producing an aerosol comprising passing anelectric arc between a counter electrode and a source material with thenet current flow being in the direction from the counter electrode tothe source material, providing enough energy in the arc to causedroplets of the source material to be ejected from it to form anaerosol, simultaneously flowing a selected gas through the region of thearc to carry the aerosol so formed away from the material, and tosolidify the droplets, the counter electrode being arranged to avoidsputtering from it.
 4. Method of producing an aerosol from a moltensource material comprising passing a direct current electric arc betweena counter electrode an an anode and the source material as a cathodewith sufficient energy to cause droplets of the source material to beejected from its surface to form an aerosol, simultaneously flowing aselected gas through the region of the arc to carry the aerosol soformed away from the material, the counter electrode being arranged toavoid sputtering from it.
 5. Method of producing a specimen thecomposition of which is representative on a microscopic scale of thecomposition of a fairly large region of a solid body comprising passingan electric arc between a counter electrode an an anode and the solidbody as a cathode with sufficient energy to cause droplets of thematerial of the body to be ejected from its surface to form an aerosol,simultaneously flowing a selected gas through the region of the arc tocarry the aerosol so formed away from the surface of the body, thecounter electrode being arranged to avoid sputtering from it.
 6. Methodaccording to claim 5 including also the step of maintaining a sufficientcurrent in the arc to melt at least a macroscopic portion of the body.7. Method according to claim 6 including the step of heating the body byelectromagnetic induction and thereby stirring the molten portionthereof.
 8. Method of chemical analysis comprising the steps ofproducing an aerosol in accordance with the method of claim 1, andanalyzing the solids portion of the aerosol so produced.
 9. Method ofmonitoring the composition of a bath of a molten, electricallyconductive material comprising producing an aerosol in accordance withthe method of claim 1, conducting it to a spectrometric analyticaldevice, and analyzing the solid particles of the aerosolspectrometrically.
 10. Apparatus for producing an aerosol from a sourcematerial comprising: a. an electrode, b. enclosure means for enclosing aregion adjacent to a selected surface of the source material, saidenclosure means enabling said electrode to be positioned within saidregion and adjacent to the surface of the source material, saidenclosure means also including inlet means and outlet means, d. aunipotential source of electric current, e. means for connecting thesource material to the cathode of said current source and said electrodeto the anode of said current source, and f. means for flowing a selectedgas through said enclosure means via said inlet and outlet means whilesaid connecting means is operative, to facilitate the striking andmaintenance of an arc between said electrode and the source material andto carry particles of the source material dislodged by the arc out ofsaid region through said outlet means.
 11. A lance for producing anaerosol from a bath of a molten material comprising: a. a tube having anopen end for withdrawing an aerosol from a region adjacent to thesurface of the molten material, said tube being electrically conductive,b. means defining an open-ended enclosure around a terminal portion ofsaid tube and extending beyond the end of said tube, c. means forflowing a gas into the enclosure defined by said enclosure means from apoint spaced from the open end thereof and outside of said tube, andthence out of the enclosure through said tube; d. means for mAkingelectrical contact with a molten material at a point spaced from saidtube when said tube with said enclosure means is placed open and firstinto the molten material, whereby a source of electric current may beconnected between said tube and the source material to produce anelectric arc between them strong enough to dislodge small particles ofthe source material from its surface, and e. said gas means beingoperative to carry particles dislodged from the source material by anarc out of the enclosure through said tube.
 12. Apparatus in accordancewith claim 11 including dynamic cooling means to enable the lance towithstand continued exposure to elevated temperatures.
 13. Apparatus inaccordance with claim 11 including a cap of an insulating, refractorymaterial covering the open end of said tube for limiting an arc strucktherefrom to the inner wall surface thereof, said cap having an aperturecoaxially aligned with said tube to permit striking the arc and passageof aerosols into said tube.
 14. Apparatus in accordance with claim 11,wherein said electrical contact means is a cylindrical conductive memberand constitutes the outer wall portion of said enclosure means. 15.Apparatus for producing an aerosol from a source material comprising: a.a tubular electrode open at one end, and having an exhaust openingspaced from said one end, b. means for supporting said electrode withsaid open end adjacent to and spaced from a source material, c.enclosure means for enclosing a region adjacent to the source materialand including said open end of said electrode, d. gas flow means forintroducing a carrier gas into the region enclosed by said enclosuremeans and withdrawing it from the region through the open end of saidelectrode, e. an unipotential source of electric current, f. means forconnecting the source material to the cathode of said current source andsaid electrode to the anode thereof, and g. said gas flow means beingeffective to sweep small particles such as may be produced by an arcenergized by said current source out of said enclosure means throughsaid electrode.
 16. Apparatus according to claim 15 including an annularinsulating shield fixed to the open end of said electrode for confiningan arc struck from said electrode to the internal surface thereof.